A polarizing effect can be obtained in glasses containing metal halide crystals such as silver halide, copper halide, or copper-cadmium halide crystals. These crystals can be precipitated in glasses including, but not limited to borosilicate glasses having compositions containing suitable amounts of an indicated metal and a halogen according to processes known in the art.
The polarizing effect is generated in these crystal-containing glasses by stretching the glass, and then exposing the surface of the stretched glass to a reducing atmosphere. The glass is placed under stress, typically at a temperature above the glass annealing temperature to elongate the glass, thereby elongating and orienting the crystals. The elongated article is then exposed to a reducing atmosphere at a temperature above about 250° C., but not more than about 100° C. above the glass annealing point. This develops a surface layer in which at least a portion of the halide crystals are reduced to elemental metal, for example, silver or copper. Examples of processes for the production of polarizing glass are described in detail in U.S. Pat. Nos. 4,304,584 and 4,479,819, the contents of which are incorporated herein by reference.
Polarizing glasses of the type described above are used to make polarizing elements that are used in equipment including but not limited to optical communication equipment, optical recording equipment, optical sensors, and optical interferometers. Optical isolators, which are used in optical communication systems, typically include a first polarizer, a Faraday rotator, and a second polarizer arranged on an optical axis in a holder. Two major uses of optical isolators are in optical transmission systems and in fiber-optic amplifiers.
One function of optical isolators is to prevent light from traveling backwards in optical transmission systems that include a light source such as a laser. An isolator is usually placed in the path of forward traveling light to prevent “back reflections” or backward traveling light. For example, a back reflection can occur in optical fiber systems when light transmitted from a light source such as a laser encounters an irregularity such as a change in refractive index between abutting materials or misalignment of optical fibers in the system. Back reflections result in reduced performance of the system and sometimes can adversely affect the transmission source, which is usually a laser. Isolators permit light traveling in only a forward direction from a light source to pass while preventing light from being reflected back to the light source. Although various methods exist for producing polarizing glass articles and optical isolators, it would be desirable to provided improved methods of manufacture.